Wide-angle lens with long back focus

Abstract

A retro-focus type wide-angle lens attains a wide angle of view of 100 degrees in terms of taking angle of view, a far exit pupil position so as to suppress the occurrence of color shading, and a long back focus which is five times the focal length, thus being sufficient for disposing a three-color-separating optical system. The wide-angle lens comprises, successively from the object side, a first lens group G 1 , having a negative refracting power, constituted by a negative first-a lens group G 1a and a positive first-b lens group G 1b , and a second lens group G 2 having a positive refracting power; and satisfies the following conditional expressions (1) and (2): 5.0< Bf/f   (1) −0.05<tan θ<0.05  (2) where f is the focal length of the whole system; Bf is the back focus of the whole system; and θis an angle formed between a principal ray directed to a screen corner portion and an optical axis.

What is claimed is:  
     
       1. A wide-angle lens having a long back focus comprising, successively from an object side, a first lens group having a negative refracting power and a second lens group having a positive refracting power; wherein said first lens group comprises, successively from the object side, a first-a lens group having a negative refracting power and a first-b lens group having a positive refracting power; said wide-angle lens satisfying the following conditional expressions (1) and (2): 
       
         
           5.0 <Bf/f   (1)  
         
       
       
         
           −0.05<tan θ<0.05  (2)  
         
       
       where 
       f is the focal length of the whole system;  
       Bf is the back focus of the whole system; and  
       θ is an angle formed between a principal ray directed to a screen corner portion and an optical axis.  
     
     
       2. A wide-angle lens having a long back focus according to claim  1 , further satisfying the following conditional expression (3): 
       
         
           −0.50 <f/f   G1 <−0.10  (3)  
         
       
       where 
       f G1  is the composite focal length of the first lens group.  
     
     
       3. A wide-angle lens having a long back focus according to claim  1 , further satisfying the following conditional expression (4): 
       
         
           −0.45 <f/f   G1a <−0.23  (4)  
         
       
       where 
       f G1a  is the composite focal length of the first-a lens group.  
     
     
       4. A wide-angle lens having a long back focus according to claim  1 , further satisfying the following conditional expressions (3) and (4): 
       
         
           −0.50 <f/f   G1 <−0.10  (3)  
         
       
       
         
           −0.45 <f/f   G1a <−0.23  (4)  
         
       
       where 
       f G1  is the composite focal length of the first lens group; and  
       f G1a  is the composite focal length of the first-a lens group.  
     
     
       5. A wide-angle lens having a long back focus according to claim  1 , wherein said first lens group comprises, successively from the object side, a first lens made of a negative meniscus lens having a convex surface directed onto the object side, a second lens made of a negative lens, a third lens made of a biconvex lens, a fourth lens made of a negative lens, and a fifth lens made of a positive lens; and 
       wherein said second lens group comprises, successively from the object side, a sixth lens made of a biconcave lens, a seventh lens made of a biconvex lens, eighth and ninth lenses constituting a cemented lens having a positive refracting power as a whole, and a tenth lens made of a biconvex lens;  
       said wide-angle lens satisfying the following conditional expressions (5) and (6):  
       
         
           0.18 <N   8   −N   9 <0.38  (5)  
         
       
       
         
           8.0 <R   13   /D   13 <30.0  (6)  
         
       
       where 
       N i  is the refractive index of the i-th lens from the object side;  
       R 13  is the radius of curvature of the sixth lens on an image surface side; and  
       D 13  is the axial space between the sixth and seventh lenses.  
     
     
       6. A wide-angle lens having a long back focus according to claim  5 , further satisfying the following conditional expression (3): 
       
         
           −0.50 <f/f   G1 <−0.10  (3)  
         
       
       where 
       f G1  is the composite focal length of the first lens group.  
     
     
       7. A wide-angle lens having a long back focus according to claim  5 , further satisfying the following conditional expression (4): 
       
         
           −0.45 <f/f   G1a <−0.23  (4)  
         
       
       where 
       f G1a  is the composite focal length of the first-a lens group.  
     
     
       8. A wide-angle lens having a long back focus according to claim  5 , further satisfying the following conditional expressions (3) and (4): 
       
         
           −0.50 <f/f   G1 <−0.10  (3)  
         
       
       
         
           −0.45 <f/f   G1a <−0.23  (4)  
         
       
       where 
       f G1  is the composite focal length of the first lens group; and  
       f G1a  is the composite focal length of the first-a lens group.